Most birds are mostly waterproof. Their feathers, aided by oil from preen glands, keep them pretty watertight. So why do birds avoid flying during rainstorms? It may have more to do with the air than with the water. Rainstorms tend to occur when atmospheric pressure is low. Air in a low-pressure system is less dense. But it’s dense air that gives birds the aerodynamic lift they need to take wing. Falling rain and high humidity make air even less dense. Many birds perch and wait out a storm. Afterward, birds once again take to the skies. 

More info and transcript at BirdNote.org. 

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